Metallic thin film piezoresistive transduction in micromechanical and nanomechanical devices and its application in self-sensing spm probes

ABSTRACT

Thin metallic films are used as the piezoresistive self-sensing element in microelectromechanical and nanoelectromechanical systems. The specific application to AFM probes is demonstrated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/010,578, filed Dec. 14, 2004, which is a continuation-in-part of U.S.application Ser. No. 10/826,007 filed Apr. 16, 2004, which claimsbenefit of priority of U.S. Provisional Application Ser. No. 60/468,452,filed May 7, 2003. This application also claims benefit of priority ofU.S. Provisional Application Ser. No. 60/562,652, filed Apr. 15, 2004.All of the above mentioned applications are incorporated herein byreference in their entirety.

The U.S. Government has certain rights in this invention pursuant toGrant No. ECS-0089061, awarded by the National Science Foundation (NSF);Grant No. F49620-02-1-0085, awarded by the United States Air ForceOffice of Sponsored Research (AFOSR); Grant No. DABT63-98-1-00012awarded by Defense Advanced Research Projects Agency (DARPA) and GrantNo. N00014-02-1-0602, awarded by the United States Navy, Office of NavalResearch (ONR).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to piezoresistive sensors formicro-electro-mechanical systems (MEMS) and nano-electro-mechanicalsystems (NEMS).

2. Description of the Prior Art

Piezoresitive displacement detection techniques are attractive in bothmicroelectromechanical and nanoelectromechanical systems (MEMS andNEMS), because they can be fully integrated and are easy to use.Applications include scanning probe microscopy, force and pressuresensors, flow sensors, chemical and biological sensors, and inertialsensors such as accelerometers and motion transducers. Most of theseapplications use p-type doped silicon layer as the sensing element.Doped silicon has a fairly high gauge factor (20˜100), but also highsheet resistance (10 kOhm/square) and therefore a relatively largethermal noise floor. Much higher 1/f noise is also expected in dopedsilicon due to its low carrier density. Additionally, fabricationprocesses for semiconducting piezoresistors, such as ion implantation ormolecular beam epitaxy, are complicated and expensive. Finally,semiconducting materials are also vulnerable to processing damage.Therefore, they are not suitable for some uses at nanoscale dimensions.

There is an unmet need for a piezoresitive sensing element formicroelectromechanical and nanoelectromechanical systems that is moresensitive and easier and cheaper to fabricate.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the invention provides a micro- or nano-mechanicaldevice comprising a movable element and a thin metal film used forpiezoresistive sensing of a movement of the movable element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are SEM images of exemplary devices according toembodiments of the invention.

FIGS. 2 and 3 are resonance response curves for the device of FIG. 1 a.

FIG. 4 is a thermomechanical noise spectrum density of the device shownin FIG. 1 b. First two modes are shown. Data is fit to Lorentz function.

FIG. 5 a is a three dimensional schematic view of a device according toembodiments of the invention.

FIG. 5 b is an SEM image of an AFM probe according to embodiments of theinvention.

FIGS. 7, 14 and 15 are schematics of testing set ups used to test thedevices of the examples of the present invention.

FIGS. 6 and 8 are plots of piezoresistive response of devices ofembodiments of the invention.

FIG. 9 is a plot of a noise spectrum from a metallic thin filmpiezoresistor.

FIG. 10 is a plot of force indentation of piezoresistive probes.

FIG. 11 a is a 3D topographical image obtained from direct tapping mode.AFM. FIG. 11 b is a 3D topographical image obtained from lock-inmeasurement of the metallic thin film piezoresistor.

FIGS. 12 a-h are side cross sectional views of the steps in thefabrication process flow for self-sensing non-contact/tapping modepiezoresistive SPM probes.

FIGS. 13 a-h are side cross sectional views of the steps in thefabrication process flow for self-sensing contact mode piezoresistiveSPM probes.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are directed to metal films used asa piezoresistive self-sensing element in micromechanical andnanomechanical systems. These systems preferably comprisemicroelectromechanical and nanoelectromechanical systems.

Microelectromechanical and nanoelectromechanical systems include deviceswith features having a size of 1 micron to 100 microns and 1 nanometerto less than 1 micron, respectively, in at least one dimension, andpreferably in two or three dimensions. Preferably, these featurescomprise movable features or elements, such as cantilevers, diaphragms,clamped beams, wires, etc. Microelectromechanical andnanoelectromechanical systems include, but are not limited to, scanningprobe microscopes (“SPM”), such as atomic force microscopes (“AFM”),force and pressure sensors, flow sensors, chemical and biologicalsensors, and inertial sensors, such as accelerometers and motiontransducers. For example, chemical and biological sensors may compriseone or more cantilevers having a surface coated with a material whichselectively binds to a chemical or biological analyte (i.e., gas orliquid analyte containing or consisting of the chemical or biologicalspecies of interest).

The term “film” includes relatively thin metal films, having a thicknessof about 100 nm to about 10 microns, thin metal films, having athickness of about 10 nm to about 100 nm, and ultra thin metal films,such as discontinuous or island type metal films having a thickness ofless than about 10 nm, as will be discussed in more detail below. Theterm “metal” includes pure or essentially pure metals and metal alloys.

The term “self-sensing” means that the metal film is used as thepiezoresistive element without requiring an external motion sensingdevice, such as a laser which is used to irradiate a cantilever ordiaphragm with radiation and a photodetector which is used to detect thereflected radiation to determine the deflection of the cantilever ordiaphragm. However, if desired, an external motion sensing device may beused in combination with the self-sensing metal film. Furthermore, themetal film is preferably used with a current source which provides acurrent across the metal film, and detector which detects the voltageacross the metal film to determine the amount (i.e., amplitude,frequency, direction, and/or any other suitable property) of movement ofthe metal film and movable element.

Strain gauge factor (“gauge factor”) is often used to express theelectrical resistance and strain relationship of a piezoresistivematerial, which is defined as dR/Rdε=(1+2v)+dρ/ρdε, where v is Poissonratio, ρ is resistivity, ε is the strain and R is the resistance. Thefirst term is merely from geometry deformation, and the second term isthe physical change of the resistivity due to the change of strain,mainly from strain mediated mean free path in material.

Even though thin metal films have a much lower gauge factor (1˜4) thansemiconductor piezoresistors, they have much smaller resistance and canbear much higher current density, and therefore yield comparable signalstrength. The lower resistance produces less thermal noise. Since metalfilms have several orders of magnitude higher carrier density thansemiconductor piezoresistors, their 1/f noise will be significantlylower. A much lower noise floor makes it possible to obtain similarresolution to silicon devices.

In comparison to semiconducting piezoresistors, metallic thin filmpiezoresistors can be fabricated at significantly reduced cost. Metalfilms of thickness from 10 nm to 10 microns can be simply evaporated orsputtered onto almost any substrate, such as Si, SiC, SiN, SiO₂, glassand even plastic materials. Processing damages to metal films areminimal. Metal films can be patterned well down to nanoscale dimensions,and batch fabricated onto devices in large arrays.

In a preferred embodiment of the invention, a gold thin film is used forpiezoresitive sensing. Those skilled in the art should obtain similarresults with other pure or essentially metals including, but not limitedto, Ag, Ni, Pt, Al, Cr, Pd, W, and metal alloys such as Constantan,Karma, Isoelastic, Nichrome V, Pt—W, Pt—Cr, etc.

In another preferred embodiment, the metal film is used to coat amovable element, such as a micron (i.e., 1 to 100 microns) or nanometer(i.e., less than 1 micron) size element. The movable element may be anyelement in a MEMS or NEMS which moves. Preferably, the movable elementcomprises a flexible, resilient element (i.e., a “flexture”), such as aresonator.

For example, the resonator preferably comprises a micron or nanometersized cantilever. However, it should be understood that the inventioncan be used with other resonators, including, but not limited to, doublyclamped beams, torsional resonators, and diaphragm resonators.Non-limiting examples of doubly clamped beam resonators, torsionalresonators and diaphragm resonators are disclosed in U.S. patentapplication Ser. No. 10/826,007, U.S. Pat. No. 6,593,731 and PCTApplication PCT/US03/14566 (published as WO/2004/041998) and itscounterpart U.S. patent application Ser. No. 10/502,641, allincorporated herein by reference in their entirety. For example, adoubly clamped beam resonator comprises a beam that is fixed on bothends, but whose middle portion is free hanging so that it can flex ormove perpendicular to its length. A torsional resonator may comprise, ina non-limiting example, a flexible diamond or polygonal shaped structuremounted at two anchor points and which can move by twisting or turningabout an axis between the anchor points, as described and illustrated inU.S. Pat. No. 6,593,731. A diaphragm resonator may comprise any plateshaped resonator which is anchored at one or more edges and whose middleportion is free hanging so that it can move or flex in one or moredirections. An example of a diaphragm resonator is a trampolineresonator.

In another preferred embodiment of the invention, the use of thinmetallic films as sensors in AFM probes is demonstrated. Processes forfabricating both self-sensing, non-contact/tapping mode piezoresistiveSPM probes and sensing contact mode piezoresistive SPM probes areprovided. As noted above, the metal film piezoresistive sensing elementscan also be used in other MEMS or NEMS devices, such as force andpressure sensors, flow sensors, chemical and biological sensors, andinertial sensors such as accelerometers and motion transducers. Theexemplary devices have an estimated displacement sensitivities of atleast 1.6×10⁻⁶/nm and force resolution of at least 3.8 fN/√{square rootover (Hz)}, which is comparable with its doped silicon counterparts, anda noise level below 1 nV/√{square root over (Hz)}.

Finally, a highly sensitive electronic down mixing readout scheme isemployed to extract the piezoresistive response from the thin metalfilms. In a preferred embodiment, this detection scheme and suitablecircuit(s) are used for metallic self-sensing piezoresistive probes forcontact mode and non-contact mode AFM operations.

Thin Metallic Films

Thin metal films have a rather low gauge factor (2˜4). Because they havemuch smaller resistance and can bear much higher current density thansemiconductor piezoresistors, they can yield comparable signal strengthto semiconductor piezoresistors. The lower resistance produces lessthermal noise. Since metal films have several orders of magnitude highercarrier density than semiconductor piezoresistors, their 1/f noise willbe significantly lower. With devices working at resonant frequency anddoing ac measurements, thin metal films can be highly sensitive.

Metallic thin film piezoresistors can be fabricated at significantlyreduced cost. Metal films can be simply evaporated or sputtered onto toalmost any substrate. Processing damages to metal films are minimal.Metal films can be patterned well down to nanoscale dimensions, andbatch fabricated onto devices in large arrays.

In the illustrated embodiment, thin films of gold are used forpiezoresistive sensing. For bulk gold, v is 0.42, and the typical gaugefactor is 1˜4. Thin gold films can be divided into three differentregions according to the film thickness. Films with thickness above 100nm are more bulk like. Films with thickness between 10 nm and 100 nmhave a continuous film regime. For a thickness below 10 nm, the film isoften discontinuous. Discontinuous film has a much larger strain gaugefactor, because of the metal island gap. In the illustrated embodiment,a 30 nm-50 nm thick gold film, which falls into the continuous thin filmregion, is used as a piezoresistive layer. However, it should beunderstood that the dimensions of the metallic thin films can varyconsiderably. For example, based on measurement results, all continuousthin gold films, from 30 nm gold films up to 10 micron film, thepiezoresistive response is on the same order of magnitude. The metalfilm may have any suitable width and length. For example, the metal filmmay be a narrow wire (such as a wire having a cross sectional area ofabout 100 nm² or less) or it may cover all or a portion of a surface ofa movable element of the device, and have a width of about 100 nm up toa 10 microns, such as 200 nm to 2 microns.

In addition to gold, a broad group of pure or essentially pure metals,including but not limited to, nickel, platinum, palladium, tungsten,aluminum etc., can also be used for piezoresistive sensing. Metalalloys, including but not limited to, Constantan, Karma, Isoelastic,Nichrome V, Pt—W, and Pd—Cr, can also be used for piezoresistivesensing. The table below lists some of the exemplary metals and thegauge factor for some of these metals.

Material Composition GF Au 100 (i.e., essentially pure or 100% Au),continuous 2.6 Au 100, discontinuous 24-48 W 100 Pt 100 Al 100 Cu 100 Cr100 Ag 100, discontinuous 45 Pd 100 2.5 Ni 100 40 Constantan 45 Ni, 55Cu 2.1 Nichrome V 80 Ni, 20 Cr 2.1 Pt—W 92 Pt, 8 W 4.0 Isoelastic 36 Ni,8 Cr, 0.8 Mo, 55.5 Fe 3.6 Karma 74 Ni, 20 Cr, 3 Al, 3 Fe 2.0 Ni—Ag 35-50Ni, Ag Pt—Cr 87 Pt, 13 Cr Armour D 70 Fe, 20 Cr, 10 Al 2.0

Resonators

As discussed above, the preferred resonator structures comprisecantilevers, such as cantilvered NEMS or MEMS structures. An SEM imageof two exemplary cantilevers are shown in FIGS. 1 a and 1 b. FIG. 1 a isan SEM image of a 10 μm long, 2 μm wide cantilever, with f₀=1.5 MHz.FIG. 1 b is an SEM image of a 33 μm long, 4 μm wide cantilever, withf₀=52 KHz. The devices have a final resistance of 150 ohm. As shown inFIGS. 1 a and 1 b, the cantilevers 1 preferably contain an opening ornotch 3 near the cantilever base 5 containing the contact pads. Theportions of the cantilevers 1 which surround the notch 3 are referred toas “legs” 7. The gold film 9 is preferably formed at least in the leg 7portions of the cantilever. If desired, the notches 3 may be omitted.

Preferably, cantilevered NEMS structures shown in FIGS. 1 a and 1 b arefabricated using a method similar to that disclosed in Y. T. Yang et al,Appl. Phys. Lett. 78, 162 (2001), incorporated herein by reference. Anexemplary method is described below. The starting material is an 80 nmthick epitaxially grown silicon carbide on a silicon wafer. Theselection of silicon carbide is more of convenience than of necessity.For example, silicon and its other compounds such as silicon nitride andsilicon oxide can be used instead. First, gold contact pads are definedby photolithography. Second, the strain concentration legs, such as thelegs 7 shown in FIG. 1 a defined by metal interconnection patterns arewritten by electron beam lithography. A 1 nm chromium adhesion layerfollowed by a 30 nm gold layer are thermally evaporated on a photoresistpattern located on silicon carbide and then lifted-off to provide adesired pattern. Following ebeam lithography, a 50 nm chromium layer ispatterned as an etching mask over the entire cantilever forming area.Then the sample is etched with an electron cyclone reaction (ECR)etcher, using a mixture of 1:1 Ar:NF₃ gas. A 250V bias voltage is usedto anisotropically etch the SiC layer. The bias voltage is thendecreased to 100 V, which makes the etching isotropic. A silicon layerbelow the device cantilever is etched, and the silicon carbide structureis released. Etching is stopped when the cantilever is barely undercut.The chromium mask is removed by wet etch to reduce the stress on thecantilever, which could cause cantilevers to curl up. A final very shortECR dry etch is used to fully release the cantilever. The sample is thenglued onto a piezoelectric ceramic (PZT) actuator, and the whole deviceis mounted onto a chip carrier, and electrical connections are made bywire bonding.

The sample is then loaded into a vacuum tank and measured in roomtemperature. The measurement system includes a half dc bridge, a batteryset as dc bias source and an ac-dc bias tee. The PZT actuator is drivenby the output of a network analyzer. After two stages of 67 dB gain, 50Ωinput and output impedance preamplifiers, the ac part of the signal isfed back into the network analyzer.

In FIGS. 2 and 3, resonance signals of the device in FIG. 1 a are shown,at different actuation levels with a constant dc bias of 50 mV. Thecantilever has two 5 μm long and 500 nm wide legs and a 5 μm long and 2μm wide pad. It has fundamental resonance at 1.5 MHz and second mode at14.8 MHz. FIG. 2 shows the resonance curve in the fundamental mode andFIG. 3 shows the resonance curve in the second mode. The insets in FIGS.2 and 3 show the peak amplitude as a function of actuation level oramplitude. In the linear response region, the amplitude at resonance isproportional to the ac signal amplitude applied to a piezoelectricactuator. In fundamental mode, the cantilever has a Q factor of 1000 invacuum. The cantilever also works in air with a lower Q of 90, as shownby the dashed lines in FIG. 2. The second mode has a quality factor ofabout 700.

These piezoresistive cantilevers are sensitive enough to sense their ownthermomechanical noise. FIG. 4 displays the noise spectrum of the deviceshown in FIG. 1 b, which has 52 kHz fundamental frequency and secondmode at 638 KHz. From the noise spectrum density and using a calculatedspring constant, the sensitivity of the cantilever can be calibrated.Apply equipartition principle to cantilever potential energy: K

z²

=k_(b)T and define sensitivity in static situation as C_(s)=ΔR/RΔz,where K is the spring constant, k_(b) is Boltzman constant, T isabsolute temperature,

z²

is mean square displacement fluctuation of cantilever, ΔR/R isresistance change ratio of the piezoresistor and Δz is the cantileverstatic displacement. An estimation can be obtained by integrating thenoise spectrum density curve to get

v²

=C_(s) ²

z²

, where

v²

is mean square voltage fluctuation. From the device geometry and elasticproperties of SiC and gold, we calculate a spring constant ofK≈0.0024N/m. Sensitivity is determined to be C_(s)=1.6×10⁻⁷/Å.Measurement is limited by Johnson and amplifier noise, with a noisefloor of 3 nV/√{square root over (Hz)} at 52 KHz and 1.4 nV/√{squareroot over (Hz)} at 638 KHz. The cantilever has a force resolution of 3.8fN/√{square root over (Hz)} at 500 KHz. It is apparent that, althoughthe sensitivity of the device is relatively low, it still has asufficient resolution due to the low noise floor.

The geometry of the device shown in FIG. 1 b may be changed to furtherimprove force resolution or displacement resolution. In the device ofFIG. 1 b, the strain sensing element 9 only occupies one tenth thelength of the cantilever. For a given spring constant, the forcesensitivity will be proportional to 1/t², so making the cantileverthinner and shorter will result in improved force sensitivity. It isnoted that doped silicon devices also suffer from large 1/f noise.Hooge's law holds that 1/f noise is inversely proportional to the totalcarrier number; therefore, making a smaller cantilever will deterioratenoise performance. While in a metal film case, the carrier number(˜10²²/cm³) is four orders of magnitude larger than that in a typicaldoped semiconductor (˜10¹⁸/cm³). 1/f noise is not a limiting issue.

In addition to cantilevers, thin metal films can also be used in othergeometries. Thus, the metal film piezoresistive sensor may be used withother resonators, including, but not limited to, doubly clamped beams,torsional resonators, and diaphragm resonators.

Device Application Examples

FIGS. 5 a and 5 b illustrate an example of the use of the metal films inself-sensing cantilever probes for atomic force microscopes. FIG. 5 a isa schematic and FIG. 5 b is an SEM image of a micromachined cantilever(i.e., probe) 11 with the following micro-scale dimensions: 150 μmlong×30 μm wide×4 μm thick. The probe 11 contains the cantilever 1, thenotch 3, base 5, legs 7, metal film 9 and a sharp AFM tip 13.Preferably, but not necessarily, the metal film 9 is formed on the sameside of the cantilever as the tip 13. The specific probe 11 shown inFIG. 5 is designed for tapping-mode AFM. Probes for contact mode AFM canbe designed as well, but with much smaller spring constants, as will bedescribed in more detail below.

FIGS. 12 and 13 illustrate the fabrication processes for bothnon-contact mode probes 11 and contact mode probes 21, respectively. Asshown in FIGS. 12 a-h, a method for making non-contact/tapping modepiezoresistive SPM probes 11 with thin metal films 9 comprises thefollowing steps.

First, as shown in FIG. 12 a, a starting substrate 101 is provided. Thesubstrate may be, for example, any suitable SOI (silicon on insulator)substrate, such as a SIMOX (i.e., oxygen implanted) or UNIBOND (i.e.,bonded) SOI substrate. The substrate may be 400 to 900 microns, such as550 microns thick, with a 0.5 to 5 micron thick oxide layer 103 betweentwo silicon portions 105, 107.

Then, as shown in FIG. 12 b, masking layers 109 and 111, which can bemade of any material usable as a mask for etching of silicon, aredeposited on both sides of the substrate 101. For example, layers 109and 111 may comprise 400 to 2000 angstrom, such as 550 angstrom thick,LPCVD deposited silicon nitride layers. Other materials, such as siliconoxynitride or aluminum oxide may also be used.

Then, as shown in FIG. 12 c, the masking layer 111 is patterned usingphotolithography (i.e., deposition/spin coating of photoresist on themasking layer, a bake of the photoresist, a selective exposure of thephotoresist, patterning of the photoresist and selective etching of themasking layer). Specifically, a crystal axis exposure pit 113 andcantilever area opening 115 are provided in the layer 111 and extendinto the silicon portion 107 of the substrate 101. The pit 113 may beformed by a KOH pit etch and the opening 115 may be formed by reactiveion etching of layer 111 using the photoresist mask.

Then, as shown in FIG. 12 d, a tip mask is formed. Preferably, the tipmask 117 is formed by photolithographically patterning the masking layer109 to leave the tip mask 117. For example, portions of layer 109 notcovered by a patterned photoresist layer may be reactive ion etched toform the mask 117. Preferably, but not necessarily, the photoresist usedin this step is removed prior to the tip etching step.

Then, as shown in FIG. 12 e, the tip mask 117 is used in the tip etchingstep. The tip 13 is formed by etching the silicon portion 105 using thetip mask 117. For example, the silicon 105 may be isotropically etchedusing KOH and be subjected to an oxidation/HF etch cycle to form the tip13. During this step, the silicon portion 105 of the substrate 101 isthinned such that its thickness is approximately equal to the desiredcantilever 1 thickness.

Then, as shown in FIG. 12 f, the metal pad and the metal piezoresistivefilm 9 are formed. Preferably, the metal film, such as a 30 to 70 nm,for example 50 nm thick Au layer is formed on the silicon portion 105adjacent to the tip 13 (i.e., the metal is preferably formed on thefront or tip side of the substrate). The metal pad and film 9 may bepatterned into a desired shape using photolithography. Alternatively,the metal pad and layer 9 may be patterned by the lift off method bydepositing the metal on a photoresist pattern and then lifting off thephotoresist pattern to leave the patterned metal on the silicon portion105 of the substrate 101.

Then, as shown in FIG. 12 g, the cantilever 1 is patterned usingphotolithography. For example, the silicon portion 105 of the substratemay be patterned using RIE or wet etching.

Then, as shown in FIG. 12 h, the back side of the substrate 101 isetched to release the cantilever 1. This may be accomplished, forexample, by a KOH etch of the back side silicon portion 107 of thesubstrate through the opening 115 in the masking layer 111, followed byan HF etch to remove the oxide 103 under the cantilever 1.

It should be noted that other materials and etching methods/media mayalso be used. Furthermore, the photoresist layers may be removed rightafter the etching step or they may be removed at a later time. Forexample, the photoresist used to form the opening 115 may be removedright after forming the opening 115 or after the step shown in FIG. 12h.

FIGS. 13 a-h illustrate a method of forming a contact modepiezoresistive SPM probe 21. In FIGS. 13 a-h, the front side of theprobe is shown on bottom rather than the top side of the probe. Ofcourse “top” and “bottom” are relative terms depending on which way theprobe is positioned and are used herein only to describe the elements inthe figures.

First, as shown in FIG. 13 a, a starting substrate 101 is provided. Thesubstrate may be, for example, any suitable semiconductor or insulatingsubstrate, such as a silicon wafer. Thus, an SOI substrate is notnecessarily used in this method. The wafer may have the same thicknessas the SOI substrate in FIG. 12 a.

Then, as shown in FIG. 13 b, masking layers 109 and 111, which can bemade of any material usable as a mask for etching of silicon, aredeposited on both sides of the substrate 101. For example, layers 109and 111 may comprise low stress 800 to 1500 angstrom, such as 1000angstrom thick, LPCVD deposited silicon nitride layers. Other materials,such as silicon oxynitride or aluminum oxide may also be used.

Then, as shown in FIG. 13 c, the back side masking layer 111 ispatterned using photolithography to form alignment holes 114 extendinginto the back side of the substrate 101. The holes 114 may be formed byreactive ion etching of layer 111 using a photoresist mask following bya KOH etch of the substrate 101 using the patterned layer 111 andoptionally the photoresist (if it has not been removed yet) as a mask.The KOH etch may comprise an etch using 30% KOH solution at 60 degreesCelsius, for example.

Then, as shown in FIG. 13 d, a membrane mask is defined in layer 111.Specifically, a membrane opening 116, which extends into the substrate,is formed in layer 111 using photolithography. The opening 116 may beformed by reactive ion etching of layer 111 using a photoresist maskfollowing by a KOH etch of the substrate 101 using the patterned layer111 and optionally the photoresist (if it has not been removed yet) as amask. The KOH etching of the substrate deepens the holes 114 until theyextend to the front side masking layer 109.

Then, as shown in FIG. 13 e, a tip mask is formed. Preferably, the tipmask 117 is formed by photolithographically patterning the masking layer109 to leave the tip mask 117. For example, portions of layer 109 notcovered by a patterned photoresist layer may be reactive ion etched toform the mask 117. Preferably, but not necessarily, the photoresist usedin this step is removed prior to the tip etching step. E-beamlithography alignment marks may also be formed during this step.

Then, as shown in FIG. 13 f, the tip mask 117 is used in the tip etchingstep. The tip 13 is formed by etching the front side of the substrateusing the tip mask 117. For example, the silicon substrate 101 may beisotropically etched using KOH. The remaining masking layer 111 is thenpreferably removed.

Then, as shown in FIG. 13 g, an optional low pressure silicon nitridelayer 118 is formed over the front side of the substrate 101 and overthe tip 13, such that the tip surface is coated with silicon nitride.Other suitable coating materials may also be used. Then the metal padand the metal piezoresistive film 9 are preferably formed directly onlayer 118 over the front side of the substrate 101. The metal film 9 maybe the same as the film 9 shown in FIG. 12 f. The metal film 9 may bepatterned using any suitable method, such as electron beam lithography,for example.

Then, as shown in FIG. 13 h, the back side of the substrate 101 isetched to release the cantilever 1. This may be accomplished, forexample, by a KOH etch of the back side of the substrate through theopening 116 in the masking layer 111.

It should be noted that other materials and etching methods/media mayalso be used. Furthermore, the photoresist layers may be removed rightafter the etching step or they may be removed at a later time.

Thus, a general method of making the probes 11 and 21 comprisesproviding a substrate; forming masking layers on front and back sides ofthe substrate; patterning the masking layers; using a patterned maskinglayer on a front side of the substrate to form an SPM tip in the frontside of the substrate; forming a patterned metal film piezoresistivesensor on the front of the substrate; and etching the substrate from theback side through an opening in a back side masking layer to form acantilever supporting the SPM tip and the metal film.

Piezoresistive Response Example

FIG. 6 illustrates a piezoresistance response of a metal film on thecantilever similar to that in FIG. 5 b according to another example ofthe present invention. The cantilever is 125 microns long, 40 micronswide and 4 microns thick with a conventional tip. The cantilever issuitable for a self-sensing probe 11 designed for tapping mode AFMapplications (i.e., this is a larger MEMS device rather than a NEMSdevice shown in FIG. 1 a). A gold thin film covers the two legs of thecantilever and forms a current loop. A very strong piezoresistanceresponse is observed, as shown in FIG. 6. Non-resonant background signalis subtracted from the raw data. The quality factor for this specificcantilever is about 220 in air. Under vacuum conditions, thepiezoresistance response is stronger, with quality factor rising above360. The data for vacuum is shifted to the right for better visualinspection.

The measurement circuit setup for this experiment is shown in FIGS. 14and 15. FIG. 14 illustrates a scheme to measure the piezoresistiveresponse of SPM probe when the probe is used in contact mode AFM. In anembodiment shown in FIG. 14, an ac bias current is passed through thepiezoresistor and ac voltage is measured to enhance the measurementsensitivity. The scheme in FIG. 14 provides a simple bridge resistancemeasurement. Furthermore, DC measurement can be directly employed todetect the bending of the cantilever. Lock-in at fixed frequency (e.g.20 kHz) can be employed to enhance the measurement sensitivity.

FIG. 15 illustrates a scheme to measure the piezoresistive response ofSPM probe when the probe is used in tapping/non-contact/ac mode AFM. Thebias current through piezoresistor is modulated at one frequency, whilethe cantilever is driven at another, different frequency. The mechanicalresponse of the cantilever is detected at their difference frequency ortheir sum frequency.

Thus, as shown in FIG. 15, an ac drive source is used to drive thecantilever 1 through a piezo drive source in the base 5. The drivesource is synchronized with an ac bias source and the outputs of the acdrive and bias sources are provided into different inputs of the mixer.The output of the mixer is provided through a low pass filter (LPF) intoa lock-in amplifier as a reference signal. The ac bias source is used tobias the metal film 9, whose output is also provided into the lock-inamplifier through another LPF and amplifier. In the scheme of FIG. 15,the ac drive source can be used to drive the cantilever at resonantfrequency, such as 240 kHz for example. Direct lock-in measurement canbe employed to detect the amplitude of the oscillation. To remove theelectrical background due to crosstalk, example shown above employs adown-mix detection scheme, which will be described in more detail below.The sample bias current may be applied at resonant frequency that is10-50 kHz higher, such as 20 kHz higher than the drive frequency (e.g.260 kHz for a 240 kHz drive frequency). Lock in measurement is performedat 20 kHz or 500 kHz, for example (see provisional application Ser. No.60/562,652, incorporated by reference for additional details).

The probe 11 is then tested with a commercial AFM system (DI dimension3100 system) equipped with a signal access module for external signalaccess and control. The measurement set up is illustrated schematicallyin FIG. 7. The standard DI probe holder is modified to facilitate thetesting of the metallic piezoresistive probes. First, the electricalconnections from the chip holder to the AFM headstage are disconnected.Second, four wires 31, 33, 35, 37 are soldered to the chip holder toenable an electrical connection to the piezo actuator 5 under the probe11 and connections to two electrical contacts pads 39, 41 on theself-sensing probe. The drive signal is applied to piezo actuator 5through an external function generator 43 (Stanford Research SystemDS345). A dc bias voltage is supplied across the two legs 7 of thecantilever 1. A resistor 43 with a resistance value similar to that ofthe cantilever is used as a balance resistor (20.3 Ohms) for extractionof resonant ac signal. The voltage change across the probe 11 is furtheramplified through a low-noise voltage amplifier 45 (Stanford ResearchSystem SR560). This oscillating ac voltage is then fed into a lock-inamplifier 47 (Stanford Research System SR830). The measurement is lockedinto the drive signal provided by the function generator. x-output ofthe lock-in amplifier is supplied to one input channel of the nanoscopecontroller through signal access module after the phase extender box.

Resonant curves from electrical measurement are first obtained. FIG. 8shows three resonant curves from the same cantilever. Trace 1 (bottomcurve) is the result from AFM built-in laser deflection measurement.Trace 2 (middle curve) is a direct electrical lock-in measurement of thecantilever with dc bias current. Trace 3 (upper curve) is an improvedmeasurement result with ac bias current (down mix scheme describedabove). Comparable signal strengths are observed in all three curves. Inoptical data, side bands due to non-flexural resonance are apparent.They are absent in the electrical measurement curves. Apparentlyelectrical measurements are immune to the shear motion displayed in theoptical measurement data. A comparison between trace 2 and trace 3 showsthat the down mix scheme can effectively eliminate the cross-talk signalthat is usually inevitable in such a measurement.

The noise spectrum measurement is then performed on the metallic thinfilm probe, as shown in FIG. 9. Very low noise spectrum is observed. Forfrequency >1000 Hz, the noise level is below 1 nV/√Hz, smaller than theJohnson noise generated by a 50 Ohm resistor at room temperature.Generally speaking, at the same frequency range, the noise level in p+silicon is about 30 nV/√Hz. The noise performance of metallicpiezoresistor is at least about 30 times better than semiconducting Simaterial.

For contact-mode AFM, the cantilever is not operated at resonance butfollows the topography of the sample surface. Cantilever's DC orquasi-DC response is of most concern for contact-mode operations. Inthis case, AFM force indentation is employed to modulate the piezo probez motion at a certain range after the cantilever is in contact withsample surface. The cantilever is bent accordingly at the modulationfrequency. The modulated piezoresistive signal is picked up by awide-band dc amplifier and measured by an oscilloscope. Data for forceindentation of the probes is shown in FIG. 10 (right axis). The voltageapplied to z-component of the piezotube is also shown for comparison(left axis). This voltage modulation corresponds to 300 nm bendingamplitude. 55 μV signal amplitude is observed across the piezoresistiveprobe. This corresponds to 0.88 mV/nm after 2000 times voltageamplification. For standard optical AFM detection, the response is 20mV/nm. Given the extremely low noise in the exemplary piezoresistor, a30 times higher gain amplifier can be used and work on comparable noisefloor with a signal response of 26.4 mV/nm, matching the performance ofthe optical cantilevers.

A standard SPM calibration grating is employed to demonstrate theimaging capability of the exemplary metallic thin film probes. Thegrating is a 1-D array of rectangular SiO₂ steps on a silicon wafer with3 micron pitch. The step height is 20 nm±1 nm. The topographic imageshown in FIG. 11 b is acquired from monitoring the output of the lock-inamplifier when AFM is operated at “lift mode”. Optical tapping mode AFMimage is present in FIG. 11 a for comparison. Even without signalconditioning, the metallic thin-film piezoresistor yields very highsignal to noise ratio. The image quality is comparable to that of theoptical measurement result. Thus, the SPM, such as the AFM is used toeither determine and/or image characteristics of a surface beingexamined by the AFM probe 11, 21 based on the piezoresistive response ofthe metal film 9. In other words, the AFM probe may be used to image asurface of a material as shown in FIG. 11 b or to determine one or morecharacteristics of the surface of a material, as may be carried out withan AFM. Furthermore, while not shown in the Figures, a data processingdevice, such as a computer or a dedicated processor, is used to processthe signal from the AFM probe and the associated equipment, such as thelock-in amplifier, to create, store and/or display the image and/or datacorresponding to the surface characteristics. A metal film has beendescribed above as the preferred piezoresistive film. However, otherpiezoresistive material films may also be used instead. For example,piezoresistive semiconductor films, such as doped silicon films, forexample p-type doped silicon films, may be formed on resonator surfacesand used to detect movement of the resonator.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andmodifications and variations are possible in light of the aboveteachings or may be acquired from practice of the invention. Thedescription was chosen in order to explain the principles of theinvention and its practical application. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

1-28. (canceled)
 29. A microelectromechanical or nanoelectromechanicalsystem comprising a) a movable element having a size from 1 nm to lessthan micron in at least one dimension; and b) a piezoresistive sensingelement configured to sense a movement of the movable element, saidpiezoresistive sensing element comprising a thin metal film, wherein,when the movable element moves in a first direction, at least a portionof the thin metal film extends in the first direction.
 30. The system ofclaim 29, wherein the movable element has a size of no more than 100microns in at least two of the three dimensions.
 31. The system of claim29, wherein the movable element is a cantilever.
 32. The system of claim29, wherein the thin metal film has a thickness from 10 nm to 10microns.
 33. The system of claim 29, wherein the thin metal film is awire having a cross sectional area of about 100 nm² or less.
 34. Thesystem of claim 29, wherein the thin metal film is a discontinuous metalfilm.
 35. The system of claim 29, wherein the thin metal film comprisesat least one of Au, W, Pt, Al, Cu, Cr, Ag, Pd and Ni.
 36. The system ofclaim 29, wherein the thin metal film comprises a metal alloy.
 37. Thesystem of claim 36, wherein the metal alloy is Constantan, Karma,Isoelastic, Nichrome V, Pt—W or Pd—Cr alloy.
 38. The system of claim 29,wherein the piezoresistive sensing element has a noise level below 3.5nV/√Hz in a frequency range 62 kHz-67 kHz or a noise level below 1.0nV/√Hz at a frequency of 5000 kHz.
 39. The system of claim 29, furthercomprising a current source configured to provide a current across thethin metal film and a detector configured to detect a voltage across thethin metal film to determine an amount of the movement of the movableelement.
 40. A microelectromechanical or nanoelectromechanical systemcomprising a) a movable element; and b) a piezoresistive sensing elementconfigured to sense a movement of the movable element, saidpiezoresistive sensing element comprising a thin metal film, wherein thepiezoresistive sensing element has a noise level in a frequency range 62kHz-67 kHz below 3.5 nV/√Hz or a noise level at a frequency of 5000 kHzbelow 1.0 nV/√Hz.
 41. The system of claim 40, wherein the movableelement has a size of no more than 100 microns in at least two of thethree dimensions.
 42. The system of claim 40, wherein the movableelement has a size from 1 nm to less than micron in at least onedimension.
 43. The system of claim 40, wherein the movable element is acantilever.
 44. The system of claim 40, wherein the thin metal filmcovers at least a portion of a surface of the movable element.
 45. Thesystem of claim 40, wherein the thin metal film is a gold film.
 46. Thesystem of claim 45 wherein the gold film has a thickness from 30 rim to10 microns.
 47. The system of claim 40, further comprising a currentsource configured to provide a current across the thin metal film and adetector configured to detect a voltage across the thin metal film todetermine an amount of the movement of the movable element.
 48. A methodof operating a microelectromechanical or nanoelectromechanical systemcomprising a movable element and a metal film piezoresistive sensingelement, the method comprising: operating the movable element at afrequency of 62 kHz-67 kHz or 5000-100,000 kHz; and determining anamount of the movement of the movable element; wherein thepiezoresistive sensing element has a noise level in the frequency range62 kHz-67 kHz below 3.5 nV/√Hz or a noise level in the frequency of5000-100,000 kHz below 1.0 nV/√Hz.